📄 csdn_文档中心_understanding strings in com.htm
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style="FONT-FAMILY: Arial; FONT-SIZE: 9pt">encapsulation of the
allocation and deallocation procedures within the constructor and
destructor; <o:p></o:p></SPAN>
<LI class=MsoNormal
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style="FONT-FAMILY: Arial; FONT-SIZE: 9pt">duplication of the
contents (through <I>CComBSTR::Copy()</I>); <o:p></o:p></SPAN>
<LI class=MsoNormal
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style="FONT-FAMILY: Arial; FONT-SIZE: 9pt">possibility to append
almost any kind of string to the wrapped BSTR exploiting the
overloading feature of C++; <o:p></o:p></SPAN>
<LI class=MsoNormal
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style="FONT-FAMILY: Arial; FONT-SIZE: 9pt">support for readable
string comparisons through the customized ! operator;
<o:p></o:p></SPAN>
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style="FONT-FAMILY: Arial; FONT-SIZE: 9pt">basic I/O operations to
store the contents of the string to, and retrieve it from, a
structured storage stream. <o:p></o:p></SPAN></LI></UL>
<P>On the other hand, MFC does not provide any direct wrapper class
for system strings. All the support is an integral part of the
extremely versatile <I>Cstring</I> class. As shown in the following
code snippet borrowed from the class's prototype in <I>afx.h,</I>
there are only a couple of methods specifically generating COM
strings: </P>
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style="BACKGROUND: #dfdfdf; PADDING-BOTTOM: 3.75pt; PADDING-LEFT: 3.75pt; PADDING-RIGHT: 3.75pt; PADDING-TOP: 3.75pt"><PRE>// OLE BSTR support (use for OLE automation)</PRE><PRE>BSTR AllocSysString() const;</PRE><PRE>BSTR SetSysString(BSTR* pbstr) const;</PRE></TD></TR></TBODY></TABLE>
<P>Internally <I>CString::AllocSysString()</I> allocates a new BSTR
using the APIs we examined in an earlier paragraph and copies its
contents to the newly created system string, which is eventually
returned to the caller. There is no such function as
CString::FreeSysString(), so to deallocate the memory occupied by
the returned BSTR, the global API <I>::SysFreeString()</I> will have
to be called. <I>CString::SetSysString()</I> instead reallocates the
BSTR pointed to by the parameter and copies its contents into it.
Both methods throw <I>CmemoryException</I> exception objects in case
of memory allocation problems. </P>
<P>Moreover, if you are using Visual C++ 5.0 or higher, you can
exploit the Direct To COM proprietary extension which includes,
among many other things, a <I>_bstr_t</I> class. The documentation
reports that it is defined inside <I>comdef.h,</I> while in reality
its declaration resides in <I>comutil.h.</I> The degree of
encapsulation and functionality is similar to ATL's <I>CComBSTR,</I>
but remember that using the COM compiler support binds you to Visual
C++ even more than ATL would do. Probably the most relevant
difference between the two implementations is that <I>_bstr_t</I>
raises C++ exceptions and thus requires your code to be prepared to
catch them, whereas <I>CComBSTR</I> does not. This detail will
likely influence your choice more than all the other possible
considerations. The following code listing summarizes the public
interface of <I>_bstr_t;</I> the comments should make it easy to
understand what the diverse method groups are up to: </P>
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style="BACKGROUND: #dfdfdf; PADDING-BOTTOM: 3.75pt; PADDING-LEFT: 3.75pt; PADDING-RIGHT: 3.75pt; PADDING-TOP: 3.75pt"><PRE>class _bstr_t {</PRE><PRE>public:</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>// Constructors</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>//</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>_bstr_t() throw();</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>_bstr_t(const _bstr_t& s) throw();</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>_bstr_t(const char* s) throw(_com_error);</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>_bstr_t(const wchar_t* s) throw(_com_error);</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>_bstr_t(const _variant_t& var) throw(_com_error);</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>_bstr_t(BSTR bstr, bool fCopy) throw(_com_error);</PRE><PRE> <o:p></o:p></PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>// Destructor</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>//</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>~_bstr_t() throw();</PRE><PRE> <o:p></o:p></PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>// Assignment operators</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>//</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>_bstr_t& operator=(const _bstr_t& s) throw();</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>_bstr_t& operator=(const char* s) throw(_com_error);</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>_bstr_t& operator=(const wchar_t* s) throw(_com_error);</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>_bstr_t& operator=(const _variant_t& var) throw(_com_error);</PRE><PRE> <o:p></o:p></PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>// Operators</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>//</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>_bstr_t& operator+=(const _bstr_t& s) throw(_com_error);</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>_bstr_t operator+(const _bstr_t& s) const throw(_com_error);</PRE><PRE> <o:p></o:p></PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>// Friend operators</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>//</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>friend _bstr_t operator+(const char* s1, const _bstr_t& s2);</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>friend _bstr_t operator+(const wchar_t* s1, const _bstr_t& s2);</PRE><PRE> <o:p></o:p></PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>// Extractors</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>//</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>operator const wchar_t*() const throw();</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>operator wchar_t*() const throw();</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>operator const char*() const throw(_com_error);</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>operator char*() const throw(_com_error);</PRE><PRE> <o:p></o:p></PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>// Comparison operators</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>//</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>bool operator!() const throw();</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>bool operator==(const _bstr_t& str) const throw();</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>bool operator!=(const _bstr_t& str) const throw();</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>bool operator<(const _bstr_t& str) const throw();</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>bool operator>(const _bstr_t& str) const throw();</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>bool operator<=(const _bstr_t& str) const throw();</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>bool operator>=(const _bstr_t& str) const throw();</PRE><PRE> <o:p></o:p></PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>// Low-level helper functions</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>//</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>BSTR copy() const throw(_com_error);</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>unsigned int length() const throw();</PRE><PRE> <o:p></o:p></PRE><PRE>private:</PRE><PRE><SPAN style="mso-tab-count: 1"> </SPAN>// [...private stuff omitted...]</PRE><PRE>}</PRE></TD></TR></TBODY></TABLE>
<P align=right style="TEXT-ALIGN: right"><B><A
href="http://www.devx.com/free/mgznarch/vcdj/1998/dec98/comstring3.asp"><FONT
color=#990033>continued...</FONT></A></B></P>
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<P align=center class=MsoNormal
style="TEXT-ALIGN: center"><SPAN
style="FONT-FAMILY: Arial; FONT-SIZE: 9pt">Copyright © 1999 -
Visual C++ Developers Journal<SPAN
style="COLOR: black"><o:p></o:p></SPAN></SPAN></P></TD></TR></TBODY></TABLE></DIV>
<P class=MsoNormal> <o:p></o:p></P><SPAN
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clear=all style="PAGE-BREAK-BEFORE: always"></SPAN>
<P><B><SPAN style="COLOR: #0e3092; FONT-SIZE: 10pt">Frameworks and
conversions </SPAN></B></P>
<P>Your ideas of OLECHAR and BSTR and your understanding of the
manner COM handles strings should be much clearer now, but we still
have to cope with type conversions to and from these somewhat
special data types and the more traditional TCHAR, WCHAR and char.
</P>
<P>ATL and MFC both use the same group of macros to deal with string
conversions. These macros' names follow a precise convention: the
characters before the "2" indicate the original type of the variable
to convert, and the characters after the "2" indicate the
destination type after the conversion. The following table lists the
valid symbols in a conversion macro name: </P>
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<P class=MsoNormal><B><SPAN
style="FONT-FAMILY: Arial; FONT-SIZE: 9pt">Short
name</SPAN></B><SPAN
style="COLOR: black; FONT-FAMILY: Arial; FONT-SIZE: 9pt"><o:p></o:p></SPAN></P></TD>
<TD
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<P class=MsoNormal><B><SPAN
style="FONT-FAMILY: Arial; FONT-SIZE: 9pt">Data
type</SPAN></B><SPAN
style="COLOR: black; FONT-FAMILY: Arial; FONT-SIZE: 9pt"><o:p></o:p></SPAN></P></TD></TR>
<TR>
<TD
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<P class=MsoNormal><SPAN
style="FONT-FAMILY: Arial; FONT-SIZE: 9pt">A<SPAN
style="COLOR: black"><o:p></o:p></SPAN></SPAN></P></TD>
<TD
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<P class=MsoNormal><SPAN
style="FONT-FAMILY: Arial; FONT-SIZE: 9pt">LPSTR, char*<SPAN
style="COLOR: black"><o:p></o:p></SPAN></SPAN></P></TD></TR>
<TR>
<TD
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<P class=MsoNormal><SPAN
style="FONT-FAMILY: Arial; FONT-SIZE: 9pt">OLE<SPAN
style="COLOR: black"><o:p></o:p></SPAN></SPAN></P></TD>
<TD
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<P class=MsoNormal><SPAN
style="FONT-FAMILY: Arial; FONT-SIZE: 9pt">LPOLESTR<SPAN
style="COLOR: black"><o:p></o:p></SPAN></SPAN></P></TD></TR>
<TR>
<TD
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<P class=MsoNormal><SPAN
style="FONT-FAMILY: Arial; FONT-SIZE: 9pt">T<SPAN
style="COLOR: black"><o:p></o:p></SPAN></SPAN></P></TD>
<TD
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<P class=MsoNormal><SPAN
style="FONT-FAMILY: Arial; FONT-SIZE: 9pt">LPTSTR, TCHAR*<SPAN
style="COLOR: black"><o:p></o:p></SPAN></SPAN></P></TD></TR>
<TR>
<TD
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<P class=MsoNormal><SPAN
style="FONT-FAMILY: Arial; FONT-SIZE: 9pt">W<SPAN
style="COLOR: black"><o:p></o:p></SPAN></SPAN></P></TD>
<TD
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